1. Field of the Invention
The present invention relates to a second-order band-pass filter and a wireless apparatus using the same, and more particularly, to a second-order band-pass filter and a wireless apparatus using the same that bends two quarter-wavelength open stubs to form a cross coupling structure for generating at least two transmission zeros.
2. Description of the Prior Art
In a RF front-end circuit of a modern communications device, a band-pass filter is an important and essential component for reducing unnecessary emission of harmonics and parasitic signals from a transmitter, or enhancing noise elimination capability of a receiver when receiving signals. Generally, an operation frequency of the modern communications device is substantially located at a microwave band, and thus it is an economical and practical approach to realize the band-pass filter on a printed circuit board (PCB) in the form of a transmission line, which has in fact been applied to the wireless communications device operated in a millimeter wave band.
However, at present, such design is seldom applied to a front-end circuit of a Wireless Local Area Network (WLAN) device, especially those operated near an ISM (Industrial, Scientific and Medical) band of the United States, mainly because a high-order microwave band-pass filter is needed to achieve a sufficient Out-Band Rejection capability. Even though the high-order microwave band-pass filter has a steep cut-off frequency response characteristic, due to its large overall length or size, it often occupies too much space in the RF circuit. Thus, it is hard to realize in a standard motherboard, and is contrary to the design demands of the modern communications device, which emphasizes lightness, thinness, smallness, etc.
Please refer to FIG. 1. FIG. 1 is a schematic diagram of a conventional second-order band-pass filter 10. The second-order band-pass filter 10 is realized on a printed circuit board, and includes a first signal terminal 11, a second signal terminal 12, a first transmission line resonator 13, a second transmission line resonator 14, and an impedance inverter 15. The first signal terminal 11 and the second signal terminal 12 are for the use of signal input and output. The first transmission line resonator 13 and the second transmission line resonator 14 are respectively coupled to the first signal terminal 11 and the second signal terminal 12, and are a pair of quarter-wavelength open stubs that are symmetric to each other and extend in opposite directions, wherein characteristic impedance and electrical length of each quarter-wavelength open stub are represented by Z1 and θ1, respectively. The impedance inverter 15 is coupled between the first transmission line resonator 13 and the second transmission line resonator 14, and includes a first micro strip line 154, a second micro strip line 156 and an inductor 152. The first micro strip line 154 has one end coupled to the first signal terminal 11, and the other end coupled to a ground GND through the inductor 152. The second micro strip line 156 is symmetric to the first micro strip line 154, and has one end coupled to the second signal terminal 12 and the other end coupled to the ground GND through the inductor 152 as well. Characteristic impedance and electrical length of the first micro strip line 152 and the second micro strip line 154 are represented by Z2 and θ2, respectively.
Since the quarter-wavelength open stub is equivalent to a series resonance circuit, and the impedance inverter 15 is utilized for providing electrical coupling and impedance matching for the two series resonance circuits, i.e. the first transmission line resonator 13 and the second transmission line resonator 14, a frequency response of the second-order band-pass filter 10 is substantially similar to that of a lumped-type second-order band-pass filter. The parameters of each transmission line (Z1,θ1, Z2 and θ2) and the size of the grounding inductor 152 can be adjusted according to required frequency responses by basic circuit analysis, and thus are not narrated herein. In addition, since the electrical coupling between the two series resonance circuits is performed by the inductor, an extra transmission zero is generated in a high frequency region of the pass-band by the second-order band-pass filter 10, so as to enhance the out-band rejection capability. However, for such second-order band-pass filters, the frequency response in a lower frequency part of the pass-band is still similar to that of a common second-order band-pass filter, which cannot satisfy out-band rejection requirements of the WLAN devices.